The present invention is directed to digitizer tablets. It particularly concerns digitizer tablets that employ sets of parallel conductors adjacent to the grid surface.
A digitizer tablet is a device for generating signals that indicate the position of a stylus on the grid surface of the tablet. A typical digitizer tablet includes two sets of parallel grid conductors disposed below and parallel to the grid surface. Each set of parallel conductors is perpendicular to the other set. Circuitry inside the digitizer tablet typically drives a coil in the stylus and senses the signals that the driven coil induces in the grid conductors. In the alternative, the grid conductors can be driven in succession and the resultant signals induced in the stylus coil can be sensed. The time of occurence of the stronger signal is then an indication of the location of the stylus.
Although the principles involved in the two types of tablets are the same and the present invention is applicable to both types, we will discuss only the type of tablet in which it is the stylus that generates the signals and the grid conductors that receive them. In this type, operational circuitry senses the signals on each grid conductor in succession and identifies the ones carrying the strongest signals. The conductors in each set carrying the strongest signals are always adjacent to each other, and the circuitry determines the ratio between the strengths of the signals on these two conductors. From this ratio, it determines the position of the stylus between those two conductors.
It is important that the conductors be as parallel to each other as possible so that the resultant signal ratios for a given set of conductors is substantially independent of the longitudinal position of the stylus along that set of conductors. As the stylus gets close to the ends of the conductors, the magnitudes of the signals for a given proximity to a set of conductors changes, but the ratios remain the same so long as the conductors remain parallel. If the conductors depart from a parallel configuration in a region in which they receive radiation from the stylus, however, the ratio will not depend exclusively on the transverse position of the stylus with respect to that set of conductors. This effect detracts from the accuracy of the tablet.
There are several ways of maintaining the required parallelism. One way is to keep the spacing between the buffer amplifiers that receive the grid-conductor signals equal to that of the grid conductors in the sensing region. This keeps the conductors, as well as any switching circuitry that occurs ahead of the amplifiers, at a uniform separation. It is not important that the circuitry beyond the amplifiers be parallel, because that circuitry typically carries higher-amplitude signals and lower impedances. However, this method of maintaining parallelism is not practical, because it requires a separate amplifier for each grid line. Typically, only one or two amplifiers are employed; the signal from only one grid line at a time is amplified, and switches that occur ahead of the amplifier determine which of the grid conductors will be coupled to the amplifier.
Another way of achieving the parallelism is to take advantage of the shield plate that is sometimes placed between the operational circuitry and the grid conductors in the sensing region. The grid conductors are formed on a flexible substrate, which is bent around the edge of the shield so that connections to the operational circuitry can be made beneath the shield. Although the conductors must depart from parallelism to be connected to the operational circuitry, this departure occurs below the shield, which ordinarily prevents radiation from the stylus from reaching the parallel portions of the conductors.
Although this arrangement is reasonably effective and avoids the need to provide a large number of amplifiers, it can still be subject to erroneous readings as the stylus is brought near the edge of the shield and some radiation therefore finds its way to the conductors beneath the shield.
One way to eliminate this problem is to employ a further shield in the form, for instance, of the type of shield box that is often required by FCC regulations in order to prevent propagation of regulated-frequency radio waves beyond the immediate vicinity of the operational circuitry. By placing the parallel portions of the conductors inside the box, exclusion of stylus-induced radiation from the parallel parts of the grid conductors can be assured. However, this type of an arrangement still has certain disadvantages. In particular, since the conductors must remain parallel at all points outside the shield box, the shield box must be made larger than the effective sensing area of the tablet if it is to provide openings large enough to allow the conductors to enter the box; the digitizer tablet thus becomes excessively large.
It is accordingly an object of the present invention to avoid the erroneous readings that can result from exposure of parallel portions of the grid conductors but at the same time to permit the digitizer size to be kept to a minimum.